Strip chart recorders are used to record changes in signals, and commonly comprise a strip of paper that is passed beneath a stylus coupled to a sensor. As a signal monitored by the sensor changes, the stylus deflects in relation to the signal change, generating markings that form a record of the sensor changes on the strip of paper. Strip chart recorders are well suited for recording continuous signals, such as temperature or another environmental condition. Changes in the signal may be observed and/or archived by removing the marked strip of paper, or a portion thereof, from the strip chart recorder.
A common mechanism used to deflect the stylus of a strip chart recorder in response to temperature is a bi-metal coil. According to one exemplary implementation of a bi-metal coil, the coil comprises two metals laminated together, each having a particular rate of expansion in response to temperature. When the coil is heated or cooled, the high expansion metal on the coil expands or contracts more than the low expansion metal of the coil, which causes the coil to move in relation to the temperature changes. The stylus may be attached to the center of the coil so that it deflects as the coil winds and unwinds. The stylus generates markings indicative of the angular relationship of the deflection on a strip chart. The chart is pulled under the stylus by a gear motor that incorporates a timing element to create a running record over time of the temperature events.
The strip chart may be reviewed to determine the temperature conditions over a recorded period. For example, a strip chart recorder may be used to record the temperature conditions of a item or group of items during transit. Such items may be sensitive to temperature or an elapsed time at a temperature, such as products intended for human use or consumption (e.g., foods and food-related products, beverages, medicines, cosmetics, etc.). By reviewing the strip chart, a user can determine whether the items have been exposed to undesirable temperature conditions (e.g., inadvertent freezing or thawing, extreme heat or cold, time spent outside of a particular desirable temperature range, undesirable temperature cycles or fluctuations, etc.).